Is that not just the remote on the key? That is what I assumed it was, correct me if I am wrong. PDIM = Personal Device Interface Module

Edit: I found this on a different forum - "Personal Device Interface Module (PDIM) allows for the use of the redundant steering wheel controls with an auxiliary device." That makes me think it might be something different, hard to tell what that definition means.

it allows your car to interface with different electronics like your phone to use for Bluetooth connectivity or allows you to control your ipod using your steering wheel controls to change and browse music on your ipod.

these are only a couple of applications that i know of, there could well be more.